Gas-phase polymerization processes are economical processes for the polymerization or copolymerization of olefins. Such gas-phase polymerization processes can be carried out either as gas-phase fluidized-bed processes or as stirred gas-phase processes. A characteristic of gas-phase fluidized-bed processes is that the bed comprising polymerizing polymer particles is kept in the fluidized state by introduction of a gas mixture from below. This gas mixture also removes the heat of polymerization from the reactor. The reaction gas is cooled in a heat exchanger located outside the reactor and is recirculated back into the reactor through a gas distributor plate (cycle gas).
However, the cycle gas also entrains a certain amount of finely divided polymer and carries it from the reactor and into the cycle gas system. These polymer particles comprise active catalyst and can thus polymerize further in the cycle gas system. If these particles deposit in the cycle gas system, deposits and fouling can occur in these places. These deposits can lead to malfunctions (blockage of the cooler, conglutination in the compressor) and parts of these deposits can become detached again. The detached deposits can block the holes of the gas distributor plate of the reactor and thus necessitate system shutdown and costly cleaning. If pieces of detached deposits get through the gas distributor plate into the reactor, the product quality is adversely affected by these particles by the formation of so-called specks or gels. Particularly in the case of products for film applications, out-of-specification material may thus be obtained.
For reducing the proportion of fine polymer dust in the cycle gas system, it is possible to install in the cycle gas line a cyclone downstream of the reactor outlet. However, complete precipitation cannot be achieved by means of a cyclone, as fine dust containing active catalyst may pass the cyclone.
Another option for avoiding fouling and the formation of polymer deposits in the cycle gas system is feeding a catalyst poison into the cycle gas line.
EP 0 431 626 A1 discloses a method for inhibiting polymer build-up in a heat exchanger during the gas phase polymerization of alpha-olefins which comprises introducing para-ethylethoxybenzoate upstream of the heat exchanger in amounts sufficient to inhibit polymer build-up. The prepared polyolefin is preferably a propylene impact copolymer.
U.S. Pat. No. 5,625,012 describes a method for inhibiting polymer build-up in a recycle line and a heat exchanger during a polymerization of one or more alpha olefins in the presence of a transition metal catalyst, which comprises introducing as an antifouling agent an alcohol having 1 to 10 carbon atoms, an alkyl or cycloalkyl ether having 2 to 20 carbon atoms, or a mixture thereof at one or more locations in the recycle gas line in an amount sufficient to inhibit polymer build-up, and discloses the preparation of EPDM rubbers while feeding isopropanol into the cycle gas line.
EP-A-0 927 724 and WO 03/042253 A1 disclose gas-phase polymerization processes in which a catalyst poison having a boiling point above the maximum temperature within the cycle gas line is fed into this cycle gas line to prevent polymer deposits in the cycle gas line. The catalyst poison is preferably an alkylamino ethoxylate.
WO 2006/107373 A1 describes a process for reducing the ultra-high molecular weight polymeric material content of a high density polyethylene produced with a bis-triarylsilyl chromate catalyst system in which a catalyst deactivator, which has a boiling point lower than the maximum temperature within the cycle gas line, is introduced into the recycle gas line.
However, feeding catalyst poisons to ethylene polymerizations carried out in the presence of chromium catalysts may adversely affect the mechanical properties of the resulting polyethylenes and sometimes an increased tendency to yellowing of the obtained polymers is observed. Moreover, adding catalyst poisons can result in an increased content of ultra-high molecular weight polymeric material in the produced polyethylene which is also detrimental to the optical properties and which may further increase the gel level. Consequently, there is still a need for a process for the preparation of ethylene homopolymers or ethylene copolymers in the presence of a chromium catalyst, which process allows preparing ethylene polymers having outstanding optical, mechanical and processing properties and a low gel content and which can be carried out with high catalyst productivity and without operational problems.